Re: [PATCH] KVM: x86/pmu: Don't retry a counter whose config was rejected

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] kvm: pmu: incomplete error check leaves retry loop for non-EINVAL errors
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commit c902c2e2a69841996cdfa4c9e356844d36d6b770
Author: Luka Absandze <absandze@amazon.de>

KVM: x86/pmu: Don't retry a counter whose config was rejected

This commit prevents KVM from infinitely retrying PMU reprogramming when the
host PMU driver rejects a configuration outright by skipping the re-arm for
-EINVAL.

Link: https://lore.kernel.org/all/20260916123315.89042-1-absandze@amazon.de/

> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index d2fd47ee5ec80..ef229faafd1e1 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -680,8 +680,14 @@ void kvm_pmu_handle_event(struct kvm_vcpu *vcpu)
>  		 * reprogram bit, i.e. opportunistically try again on the next
>  		 * PMU refresh.  Don't make a new request as doing so can stall
>  		 * the guest if reprogramming repeatedly fails.
> +		 *
> +		 * -EINVAL means the event's config was rejected outright and
> +		 * can never succeed on retry, so don't re-arm; the guest can
> +		 * still do so itself by rewriting the event selector.
>  		 */
> -		if (reprogram_counter(pmc))
> +		int r = reprogram_counter(pmc);
> +
> +		if (r && r != -EINVAL)
>  			set_bit(pmc->idx, pmu->reprogram_pmi);

[Severity: High]
Does this condition fail to exclude other permanent error codes like
-EOPNOTSUPP or -ENOENT?

If a guest programs an invalid configuration (such as a PEBS precise_ip level
that the host hardware does not support), x86_pmu_hw_config() can return
-EOPNOTSUPP. 

In that scenario, reprogram_counter() will propagate this error, the condition
here evaluates to true, and KVM will incorrectly re-arm the reprogram_pmi bit.
On subsequent PMU refreshes, KVM will repeatedly retry the heavy
perf_event_create_kernel_counter() path.

Could this leave the retry loop open for other error codes, allowing an
unprivileged guest to cause high CPU overhead on the host?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260918160006.61816-1-absandze@amazon.de?part=1